1. Field of the Invention
The present invention generally relates to lifting machines and more particularly, but not exclusively, to travelling cranes intended for handling delicate loads.
2. History of the Related Art
In this type of operation, a cable is used, of which one end is wound on a drum while its opposite end is anchored to the carriage of the handling machine, the cable passing on the one hand around an idle pulley mounted to rotate above the point of anchoring, and, on the other hand, around the idle pulley of a mobile block with hook to which the load is hooked, for example by means of a grab. Between the idle pulley and the block, there is inserted in the cable a scale associated with means for stopping the control of rotation of the drum if the cable is subjected to overload or underload. The former occurs if the load encounters an obstacle while it is being lifted, while underload occurs if the load encounters an obstacle during its descent.
The electrical devices used in connection with the scale and the drive motor of the winch present excessive response times to allow an immediate stoppage of the displacement of the load both in the rising and in the descending direction, with the result that the load may be damaged when a mishap occurs.